Preheat oven to 350F. Spray a large muffin pan with cooking spray or line with muffin liners. I prefer muffin liners, as it gets not so messy to clean after.

In a large bowl, add and whisk together dry ingredients: flour, sugar, cocoa, baking powder, baking soda. Make a well in the center of the dry ingredients mix and add the eggs, sour cream, oil and Nutella. Stir together until just combined. The mixture will be quite thick at this point.

Fold in the chocolate pieces and chips. Use a spoon or ice cream scoop to fill muffin cups 3/4 full.

Bake in preheated oven for almost 20-25 minutes until perfectly baked (insert a toothpick in the center of a muffin and if it comes out clean, they are baked). Remove from the oven and allow to cool down on wire rack before serving them.